Electric shock defibrillation is a proven technique of treatment of the serious and immediately life-threatening condition of ventricular fibrillation. For patients known to be at risk, an implantable defibrillator may be used. Such devices contain an energy source, an electrode lead system in contact in the heart, a sensing system to detect the onset of fibrillation, and a pulse generator for delivering the defibrillation shock.
Existing devices are generally designed or programmed to deliver a shock or series of shocks at a fixed interval or intervals following the detection of the fibrillation, unless fibrillation spontaneously terminates on its own first, or until recovery is achieved, as evidenced by the resumption of normal ventricular rhythm. The amount of energy to be delivered in a shock must be carefully chosen. If too small, it may not be successful in terminating the fibrillation. On the other hand, the shock must not be so large that it causes damage to the myocardium. The device generally is designed in consideration of the limited energy storage in an implanted device.
Ventricular electrical signals may exhibit a pattern, known as "fine ventricular fibrillation" during ventricular fibrillation. The fine ventricular fibrillation is characterized by relatively low signal amplitude and lack of organized features. The ventricular electrical signals may also exhibit a pattern known as "coarse ventricular fibrillation," characterized by intervals of relatively higher amplitude, which may repeat, separated by fine ventricular fibrillation intervals. It has also been suspected that it is easier to defibrillate coarse ventricular fibrillation than fine ventricular fibrillation. Because of this, previous works have suggested the possibility of timing of defibrillation shocks to features of the ventricular fibrillation waveforms as a way to improve defibrillation efficacy. However, it has not been clear from such prior works, which features are important, and how to detect and coordinate to them. A need, therefore, exists in the art for a system that improves defibrillation therapy by using the minimal amount of energy necessary to bring about effective and efficient defibrillation.